1. Field of the Invention
This invention relates to a porous coordinatively unsaturated metal complex in which a number of metal complex units are connected one another by connecting metals, and a process for producing such a metal complex. Further, this invention pertains to a compound usable as a metal complex unit and an organic ligand as component substances constituting the porous coordinatively unsaturated metal complex.
2. Description of the Related Art
Metal complex is a composite of a ligand and a metal. Metal complex exhibits high catalytic activity, and provides an optimal field for reaction. Metal complex exhibits such properties based on the premise that a metal having high reactivity and accordingly being labile stays stably in a metal complex and that a ligand has a specific conformation. Since metal complex has high usability, study concerning metal complex has been conducted in many fields.
For instance, there is known an approach regarding a “coordinatively unsaturated metal complex” in which a metal capable of making coordination bonding at plural positions or sites is supported in a complex in a so-called “coordinatively unsaturated state”, namely, in a state that a ligand is not coordinatively bound to the positions or sites where it is supposed to be coordinatively bound. Such a coordinatively unsaturated metal complex exhibits high chemical reactivity because the metal in the complex is coordinatively unsaturated. That is, it is inherently difficult for the metal to stay stably due to its high reactivity. However, the metal can be rendered stable in the complex in a coordinatively unsaturated state by devising a ligand, thereby making it possible to provide the metal complex with high catalytic activity.
Coordinatively unsaturated metal complexes having voids are described in (1) B. Chen, et al., “Cu2(ATC).6H2O: Design of Open Metal Sites in Porous Metal—Organic Crystals (ATC:1,3,5,7-Adamantane Tetracarboxylate)”, J. Am. Chem. Soc., 122, pp. 11559–11560 (2000) and (2) M. Eddaoudi, et al., “Modular Chemistry: Secondary Building Units as a Basis for the Design of Highly Porous and Robust Metal—Organic Carboxylate Frameworks”, Acc. Chem. Res., 34, pp. 319–330 (2001). These references suggest that the coordinatively unsaturated metal complexes are usable in storage of specific compounds or the like.
However, according to the above paper (1), whereas the metal complex is capable of readily incorporating CH2Cl2, it is incapable of readily incorporating C6H6, CCl4, C6H12. This shows that the substrate intended to be incorporated in the metal complex recited in these papers (1) and (2) is limited to low-molecular weight compounds.
And in the reference (3) S. Noro, et al., “New microvoid coordination polymer affording guest-coordination sites at channel walls, Chem. Comm., pp. 222–223 (2002), a porous metal complex in which a coordinatively unsaturated metal is coordinatively bound to a wall of a polyhedron defining a cavity of the metal complex instead of a vertex of a polyhedron.
But reference (3) recites that the size of the cavity in the metal complex is 15×5 Å, and accordingly, it can be concluded that the cavity has a slit-like shape rather than a void. Therefore, the metal complex having such slit-like cavities fails to incorporate a general compound as a substrate, although it can incorporate a low-molecular weight compound. Actually, what is recited in reference (3) as substrate molecules (compounds to be incorporated) is nothing more than dimethylformamide (DMF) and water molecules.
As mentioned above, academic approaches have been conducted primarily in the field of porous metal complexes because the configuration of porous metal complexes has provided intriguing topic for the researchers. There has also been proposed a technology of actually applying such a metal complex. For example, (4) Japanese Unexamined Patent Publication No.9-227571 discloses a metal complex having cavities of a predetermined size and capable of incorporating and holding specific molecules. The metal complex disclosed in reference (4) has cavities of a predetermined size resulting from its multi-layered structure constructed such that layers having a specific structure are linked one over another by way of component substances, and thus exhibits properties capable of incorporating specific molecules into the cavities. However, since the cavities of the complex are extremely narrow, the metal complex can incorporate merely small molecules such as oxygen and methane gas. Furthermore, since a metal ion as a component substance is in a so-called “coordinatively saturated state”, the metal complex incorporated with such a metal ion fails to exhibit high catalytic activity.
In addition to the aforementioned drawbacks, some of the conventional porous metal complexes are constructed by hydrogen bonding. Accordingly, they fail to provide rigidity such as stability.
There have been known organic ligands of salen type, as component substances of a metal complex. For instance, a ruthenium-based coordinatively unsaturated complex of salen type is disclosed in (5) C—C. Cheng, et al., “Novel Water-Soluble 4-4-Disubstituted Ruthenium (III)-Salen Complexes in DNA Stranded Scission, J. Chin. Chem. Soc., 45, pp. 611–617 (1998). This reference (5) discloses that the metal complex breaks DNA chain along with hydrogen peroxide. However, reference (5) neither discloses nor remotely suggests forming a porous metal complex with use of an organic ligand of salen type.
Despite the aforementioned knowledge, the prior art fails to find out a porous coordinatively unsaturated metal complex that enables to incorporate not only low-molecular weight compounds but also general compounds as substrates and enables to exhibit high catalytic activity.